The present invention relates to improved bentonite barrier compositions, and more particularly, to the use of geosynthetic clay liners comprising these improved bentonite barrier compositions having enhanced low permeability over time in containment applications.
Various materials and procedures have been developed and utilized to form low permeability barriers in containment applications. For example, low permeability barriers are needed to separate waste fluids from contaminating the surrounding environment in fly-ash repositories, industrial mineral and metal mining sites, and landfill sites. These barriers are also useful for aqueous containment applications such as leachate ponds, retention ponds, and water storage reservoirs. The term “containment” when used herein refers to both aqueous containments (e.g., ponds) as well as other containments that have components that are better separated from the surrounding environment (e.g., fly-ash repositories). For example, “containment” may refer to the separation of ponds of liquid waste streams from industrial processes or leachates produced from these or other industrial processes from the surrounding environments. A “leachate” as that term is used herein refers to an effluent containing contaminants, produced from water (e.g., rain/storm water) percolating through a depository (e.g., a landfill, a fly-ash repository, etc.). A leachate usually contains a high concentration of electrolytes as compared to fresh water.
Clay materials, such as bentonite, have been used as low permeability barriers in containment applications. Bentonite is an aluminum phyllosilicate whose composition can vary in its dominant elements. When first mined or extracted, for example, sodium bentonite mined from Wyoming, often has a moisture content that is approximately about 30% to about 35% by weight. In many instances, this moisture may be removed to be about 6% to about 15% by weight. This is considered by the industry to be “dry” bentonite despite the significant moisture content. The moisture content may vary from application to application, and may be dependent on exposure to fluids in the ground that hydrates the bentonite to a higher moisture content.
Bentonite barrier compositions are often formulated from natural or sodium exchanged bentonite and mixed with common fluid additives. In many cases, the bentonite barrier compositions may be engineered from granular Wyoming natural sodium bentonite with the additives. The granularity or the relative particle size distribution, often described in terms of mesh size in the art, can determine how well the bentonite is packed and its ease of handling. A common use of bentonite geosynthetic clay liners is to line the base of landfills to prevent the migration of leachate and/or solutions containing high concentrations of electrolytes.
While bentonite is highly absorbent, able to absorb water several times its dry mass, aqueous fluids having complex chemistries can adversely affect its absorbency. These complex chemistries often involve electrolytes that may include, but are not limited to, cations and anions such as calcium, magnesium, potassium, iron, zirconium, lead, cobalt, copper, tin, silver, sulfates, chlorides, fluorides, bromides, and the like. The composition of the electrolytes may vary based on the source material of the containment (e.g., coal source for a fly-ash repository).
Bentonite can also be used in conjunction with a geosynthetic layer to form a geosynthetic clay liner. This technique may allow for convenient transport and installation of the bentonite, and greatly reduces the amount of bentonite required. The primary indicator of the effectiveness of a liner is “permeability.” As used herein, the term “permeability” refers to the rate of flow of a fluid through a porous media (e.g., a clay liner) as measured in terms of cm/s. These barrier compositions should meet the permeability specification set by regulations (e.g., local, international, state and federal standards, etc.). It is desirable for a liner to be less permeable (i.e., have lower permeability) so that less materials are transported through the liner to the surrounding environment.